1. Field of the Invention
This invention relates to a photochemical vapor deposition apparatus.
2. Description of the Prior Art
Recently, there are studied methods for forming a vapor-deposited film of amorphous silicon for use in the photosensitive drum of a duplicating machine or a solar cell. On the other hand, a vapor depositing method is further utilized in the formation of diverse insulating films or protective films, and a variety of vapor depositing methods have been proposed in answer to various uses. Among these methods, a photochemical vapor depositing method utilizing a photochemical reaction is being now particularly watched because of having such advantages that the film-deposition rate is remarkably high and a uniform film can be formed on a portion of large area of a substrate, too.
A conventional chemical vapor depositing method utilizing a photochemical reaction comprises placing a substrate in an air-tight vessel made of material through which ultraviolet rays can be fully transmitted, feeding a photoreactive gas to flow through the vessel and applying ultraviolet rays radiated from an ultraviolet discharge lamp outside of the vessel through the wall thereof onto the substrate so that a photochemical reaction is caused to decompose the photoreactive gas and the resulting reaction product is vapor-deposited onto the substrate. In spite of having the above-mentioned remarkable advantages, this conventional photochemical deposition method that may be called "outer discharge type", has however been found to have such a defect that the reaction product is also vapor-deposited on the inner wall of the vessel, with impeding the transmission of ultraviolet rays seriously.
Thus, a photochemical vapor deposition apparatus, that may be called "inner discharge type", has been studied and developed. In the apparatus of this type, a reaction space and a discharge space are air-tightly surrounded by the same one vessel. The reaction space forms a passage for a photoreactive gas and in this reaction space a substrate is to be placed. In the discharge space, electric plasma discharge is generated and ultraviolet rays radiated from the plasma are applied onto the substrate to cause a photochemical reaction of decomposition of the photoreactive gas. Between the plasma and the substrate, there is no partition member that impedes passing of the ultraviolet rays.
In the photochemical vapor deposition apparatus of an inner discharge type, a substrate is horizontally placed on the bottom of the vessel and discharging electrodes are arranged in such a state that they are opposite to each other in a horizontal direction with the discharge space therebetween, whereby plasma discharge is generated in the horizontal direction. Since the plasma of electric discharge generated between the electrodes diffuses or expands mainly in the direction perpendicular to the discharging direction, however, there is a fear that the diffused or expanded plasma will damage the vapor-deposited film on the substrate placed below. In order to prevent such damaging of the film, it is required that the substrate be placed at a position more remote from the plasma than the mean free path of ions or electrons in the plasma. On the other hand, in order to increase the intensity of ultraviolet rays applied onto the substrate to improve the efficiency of the vapor deposition, the substrate should be kept near an ultraviolet ray source as much as possible. After all, in the photochemical vapor deposition apparatus of an inner discharge type, vapor depositing with sufficiently high film-deposition rate is not carried out, because the applying efficiency of ultraviolet rays is lowered due to the placement of the substrate at a position remote enough from the ultraviolet ray source, namely the plasma of the discharge.